fuel filter-mk3: DIY change or priming, AHU Jetta or 1Z Passat TDI engine

Apr 17, 2017
fuel filter-mk3: DIY change or priming, AHU Jetta or 1Z Passat TDI engine
  • VW TDI fuel filter draining or changing and priming the fuel system
    difficulty: 1/5
    for 1998-2005 mk4 TDI see 1000q: mk4 fuel filter, for 2005.5-2010 TDI see 1000q: mk5 Jetta/golf fuel filter change


    This DIY shows how to replace the mk3 VW Jetta TDI or VW Passat fuel filter. The replacement interval is 20,000 miles.
    Your owner's handbook and service manual calls for draining the fuel filter of water every 10,000 miles and changing it every 20,000 miles. This means install a new filter every 20,000 miles and drain it halfway through it's life. This article gives tips on changing or draining the fuel filter and then priming the fuel system.

    If you are using high percentages of biodiesel such as 85% or 100% biodiesel, you should expect to change it early since biodiesel will clean out the old buildup and clog the fuel filter if there was significant build up in the fuel system. As a safe rough estimate, 1000 miles and then again at 5000 miles, your mileage may vary with biodiesel. A more economical idea is to install a small clear inline fuel filter before the main fuel filter to filter out larger particles and let you see how clogged the fuel filter may be.

    Early symptoms of a clogged fuel filter are stumbling at high rpms or lack of power. A general lack of power could also be limp mode, see 1000q: TDI limp mode for possible causes and the solution. Make sure that these symptoms are not caused by an air leak in the fuel line or a clogged pickup at the fuel tank. Bacterial or algal growth in the fuel tank can clog the pickup. Your car has no in tank electric fuel pump, only later TDI had these.

    Fuel filter basics
    There is a temperature sensitive recirculation valve on the return line. When below 59oF, the valve recirculates return line fuel to the filter and help regulate the fuel temperature. When the fuel is above 88oF, it should switch and recirculate fuel back to the fuel tank. The return line fuel is warmer than outside temperature because it's heated by the pressurization and compression from the injection pump and ambient heat from the engine and fuel lines. If this valve or any of the fuel lines are not sealed well, it will let air bubbles into the fuel lines and this could result in engine stumbling or a hard/no start condition. Make sure the o-ring and fuel line clamps are seated properly. The arrow on the recirculation control valve should point toward the fuel tank to the rear.

    You may hear that the fuel filters are heated, this is true only in the sense that they are slightly warmed by the return line fuel. There is no active or electric heater unless someone installed an aftermarket heater for a veggie or grease system and you wouldn't want diesel or biodiesel to be too hot anyways.

    Although you are supposed to drain water every 10,000 miles, I have never seen any water in my fuel. Water from condensation would be present in fuel that has sat for a long time or that has contamination from the gas station. The water drain drains the whole filter out the bottom when unscrewed.

    Parts (click links to compare current prices)

    vinyl/latex/nitrile (fuel resistant) gloves
    chemical resistant cup for holding diesel fuel
    paper towels to catch any spilled fuel
    1 Fuel Filter (see below for part number)

    1996-1999 mk3 Jetta/Passat TDI fuel filter: VW# 1h0 127 401 e (from kermaTDI), from idparts
    replacement fuel filter T from kermaTDI

    TDI fuel filter replacement procedure

    Safety disclaimer - you are working with open fuel lines and fuel vapors when you change the fuel filter! Make sure that there are no sources of ignition, spark, or open flames near the car or where fuel vapors could reach. Work only in a well ventilated area where any fuel vapors can be immediately evacuated and if fuel is spilled, clean it up before you continue working. Although diesel vapors are not as flammable as gasoline vapors at room temperature and pressure (as seen in the video below at the 1:00 minute mark - it's a demonstration only do not try that yourself!), you still want to comply with all cautions in your factory service manual. Wear eye protection at all times when working on your car. See the TOS Agreement for the full legal disclaimer. Diesel fuel will melt asphalt and rubber lines on your car so clean up any spills immediately. Make sure to use gloves because diesel fuel has a strong odor and you don't want it soaked into your hands.

    Fuel filter circled in yellow.

    Draining the fuel filter
    The diesel filter has a water drain at the bottom. It's purpose is to let you drain out any collected water. Drain it every 10,000 miles. Just place a catch cup underneath the drain at the bottom and turn the white knob to open it. See below for a close up of the drain. Because of the quality of diesel fuel sold in North America, it's rare to have any water present and you will probably not see any water collected in a daily driven car. Water can condense in storage tanks or cars that are never driven.

    Replacing the fuel filter
    To replace, just loosen the screws that hold it and remove the clamps that hold the fuel lines. The fuel lines may be somewhat stuck to the filter line so try twisting the line before pulling it off to break the seal.

    Below left is a picture of the fuel filter. Below right is another picture showing the mounting bolts (you don't need to loosen those to remove the filter) and the fuel lines. Note the white water drain screw at the bottom of the filter. If you have a 1996 passat TDI, you have an extra T in the fuel line. You can ignore it during this procedure or see 1000q: 5th inj for more details.

    [​IMG] [​IMG]

    Pull out the filter and replace the o-ring on the thermostatic T. Make sure to replace them or else excess air bubbles can cause fuel supply problems. New ones should come with the new filter. Wipe up any fuel spills and make sure that while you are doing this procedure, that there are no possible sources of ignition or open flames anywhere near the car and adequate ventilation to clear any fuel vapors. Make sure to wad a paper towel over the fuel line when you remove it. The fuel lines should not be under pressure but some fuel may spill out.

    When you replace the fuel filter, fill it up with diesel fuel to minimize priming time. You can also use diesel purge or diesel power service. These are mostly diesel fuel anyways and they will also clean the system. Here's a video showing some of the procedure on a similar engine.

    When you replace the fuel filter, fill it up with fuel to prevent a dry start (see more on priming below).

    Here's a video summarizing the procedure on a similar engine:

    Here is what's inside the fuel filter housing, click to enlarge the thumbnails.


    Priming the fuel system or purging air (if your TDI engine can't start if you changed the filter)
    To prime the pump, first fill the fuel filter and fuel lines with fuel. If you didn't change the injectors or do something that emptied the fuel lines, just filling the filter should be enough. If you did empty the fuel lines and rail, prime the system fully. The injection pump is self priming but it'll take forever to start if the fuel lines are dry. This is hard on the battery and can overheat the starter. By priming, most of the air is removed and the engine should start with minimal cranking. If the fuel lines weren't run dry, just fill the filter and leave it at that.

    Apply suction on the return line at the fuel filter or fuel return line until fuel has circulated through the pump. You can use the small 4 mm braided hose connecting the top of the injection pump to the #4 fuel injector or the return fuel line T shown below. The problem with loosening the filter line at the filter is that the plastic T can become brittle and snap off. Remember, twist the hoses to break the seal before pulling it off. Don't bother using your mouth, you must use a vacuum pump to generate enough force. A vacuum brake bleeder or a "mity vac" hand pump will work.

    Some people loosen the fuel hard lines at the injectors to purge air. This also works fine but if you just removed the fuel injectors, counterhold the injector bodies to avoid twisting their hold-downs.If you didn't, they'll be frozen in place and won't need to be counterheld. Wrap a rag and around the line to prevent fuel being sprayed everywhere. Do not use your hand to hold the fuel since the diesel is under pressure. Although you only see "instant death" fuel pressure at the injector nozzle (in the cylinder and not exposed by loosening the fuel line), I would still avoid exposing bare hands to any pressurized fuel.

    Replace all the clamps and check for any loose tools, paper towels, etc. Take a test drive and check for any leaks. There may be many bubbles initially because of air pockets still trapped somewhere in the system. If there are still lots of bubbles after a day, check for leaks at the lines, the o-rings, the T, or at the fuel pickup in the fuel tank. Feel free to share your experience in the myturbodiesel.com VW TDI forums or if you need more help.